1. Field of the Invention
The present invention relates to a laminate type multi-semiconductor laser device and a laser beam scanning optical apparatus employing this semiconductor laser device, and in particular, to a laminate type multi-semiconductor laser device and a laser beam scanning optical apparatus employing this semiconductor laser device to be assembled into a laser printer or a digital copying machine for executing image printing at a high speed by means of a plurality of laser beams.
2. Description of Related Arts
As a laminate type multi-semiconductor laser device, there has been conventionally known one as disclosed in Japanese Patent Laid-Open Publication No. 6-244496. In this laminate type multi-semiconductor laser device, a plurality of light emitting regions are provided at specified pitches in an identical active layer.
In general, a laminate type multi-semiconductor laser device cannot wholly convert a laser drive power into an optical energy no matter whether a plurality of light emitting regions are provided in an identical active layer. The laser drive power that has not been converted into the optical energy is converted into a thermal energy as a thermal loss, thereby heating the light emitting regions. On the other hand, if a relationship between the laser drive power and a laser beam output is expressed by a function, the temperature of the light emitting regions is included as a parameter.
Therefore, when the light emitting regions are located close to one another in the laminate type multi-semiconductor laser device, due to thermal crosstalk between adjacent light emitting regions, the heat of one light emitting region is transmitted to the other light emitting region, thereby causing a problem that the absolute light quantity of the laser beam emitted from each light emitting region varies or a problem that a relative light quantity difference is generated between the laser beams emitted from the light emitting regions. Particularly, if a relative light quantity difference is generated between the laser beams emitted from the light emitting regions, the resulting image quality is significantly deteriorated to a disadvantage. Furthermore, due to the heating of each light emitting region, the wavelength of the laser beam emitted from each light emitting region changes, thereby causing a problem that a chromatic aberration is increased on a scanning surface. Furthermore, it has been required to secure a fairly large space for the provision of signal lines of the light emitting regions.
For the above reasons, in the prior art laminate type multi-semiconductor laser device it has been hard to locate the light emitting regions close to one another, and it has been perceived that the interval between light emitting regions is required to be at least about 100 .mu.m. Therefore, when the prior art multi-semiconductor laser device in which a plurality of light emitting regions are provided in an identical active layer is mounted on a laser beam scanning optical apparatus with the light emitting regions arranged in a sub-scanning direction and parallel writing is executed by this laser beam scanning optical apparatus, there results in simultaneous exposure of a plurality of lines located apart by a beam interval of several lines in the sub-scanning direction on the scanning surface. More specifically, when the interval between the light emitting regions is about 100 .mu.m, there results simultaneous exposure of a plurality of lines located apart by a beam interval of about 3 mm on the scanning surface.
When the beam interval is large in the sub-scanning direction on the scanning surface as described above, a main scanning line curves in a main scanning direction on the scanning surface to generate a bow of a considerable curve, and this significantly varies the beam interval in the sub-scanning direction for every field angle in the main scanning direction. Furthermore, if the beam interval on the scanning surface is large and when beam scanning is executed on a photosensitive drum as in a printer, the interval between projection positions of the beams significantly varies due to unevenness in rotating velocity and eccentricity of the photosensitive drum. The above results tell the fact that the laser beam scanning optical apparatus employing the prior art laminate type multi-semiconductor laser device has had the tendency of causing deterioration in the image quality.
Accordingly, as a countermeasure to the above disadvantages, as shown in FIG. 8, there has been proposed a method of narrowing the interval between light emitting regions 53a and 53b taking advantage of the fact that a pitch P in the sub-scanning direction is apparently reduced to P=P.sub.O sin.theta. by inclining, at an angle .theta., the multi-semiconductor laser device 51 in which the plurality of light emitting regions 53a and 53b are provided as located apart by P.sub.O in an identical active layer 52. In this case, the multi-semiconductor laser device 51 is to be mounted on a laser beam scanning optical apparatus of a multi-beam system in such a manner that its direction parallel to the active layer 52 is directed approximately in the main scanning direction and its direction perpendicular to the active layer 52 is directed approximately in the sub-scanning direction. Then, each laser beam emitted from the semiconductor laser device 51 has an elliptic intensity distribution in which its divergence angle is wide in the direction perpendicular to the active layer 52 and is narrow in the direction parallel to the active layer 52. Therefore, the direction of the wider divergence angle is arranged approximately in the sub-scanning direction. It is to be herein noted that Ea and Eb in FIG. 8 indicate equi-level lines of the intensity distributions of the laser beams.
On the other hand, as shown in FIG. 9, in the case of a semiconductor laser device 61 in which only one light emitting region 63 is provided in an active layer 62, the semiconductor laser device 61 is generally mounted on a laser beam scanning optical apparatus of a single-beam system in such a manner that its direction perpendicular to the active layer 62 is directed in the main scanning direction and its direction parallel to the active layer 62 is directed in the sub-scanning direction. This arrangement is adopted for the reason that the focal distances of a collimator lens and a cylindrical lens can be reduced for the compacting of the device when the direction of the wider divergence angle is arranged in the main scanning direction in terms of the construction of the laser beam scanning optical apparatus of the single-beam system. The arrangement is adopted also for the reason that there is no need for placing an optical member such as a beam expander for changing the laser beam diameter on the optical path, allowing the device construction to be simplified.
For the above reasons, the laser beam scanning optical apparatus of the single-beam system employing the semiconductor laser device 61 in which only one light emitting region 63 is provided in the active layer 62 and the laser beam scanning optical apparatus of the multi-beam system employing the semiconductor laser device 51 in which the plurality of light emitting regions 53a and 53b are provided in the identical active layer 52 have been required to employ different optical system constructions. In concrete, both the systems have been required to have different focal distances of the collimator lens, cylindrical lens and scanning lens. If an optical system is commonly used, both the systems have significantly different beam diameters on the photosensitive surfaces both in the main scanning direction and the sub-scanning direction. As a result, the laser beam scanning optical apparatus of the single-beam system and the laser beam scanning optical apparatus of the multi-beam system cannot be commonly used, and this has led to the problem of manufacturing cost increase.